Integrating Photovoltaic Systems in Power System: Power Quality

Hindawi Publishing Corporation
International Journal of Photoenergy
Volume 2014, Article ID 321826, 7 pages
http://dx.doi.org/10.1155/2014/321826
Review Article
Integrating Photovoltaic Systems in Power System: Power
Quality Impacts and Optimal Planning Challenges
Aida Fazliana Abdul Kadir,1 Tamer Khatib,2 and Wilfried Elmenreich2
1
2
Industrial Power Engineering, Faculty of Electrical Engineering, Universiti Teknikal Malaysia Melaka, 75300 Melaka, Malaysia
Institute of Networked & Embedded Systems/Lakeside Labs, Alpen-Adria-Universit¨at Klagenfurt, 9020 Klagenfurt, Austria
Correspondence should be addressed to Tamer Khatib; tamer [email protected]
Received 2 May 2014; Accepted 7 August 2014; Published 17 August 2014
Academic Editor: Hui Shen
Copyright © 2014 Aida Fazliana Abdul Kadir et al. This is an open access article distributed under the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
This paper is an overview of some of the main issues in photovoltaic based distributed generation (PVDG). A discussion of the
harmonic distortion produced by PVDG units is presented. The maximum permissible penetration level of PVDG in distribution
system is also considered. The general procedures of optimal planning for PVDG placement and sizing are also explained in this
paper. The result of this review shows that there are different challenges for integrating PVDG in the power systems. One of these
challenges is integrated system reliability whereas the amount of power produced by renewable energy source is consistent. Thus,
the high penetration of PVDG into grid can decrease the reliability of the power system network. On the other hand, power quality
is considered one of the challenges of PVDG whereas the high penetration of PVDGs can lead to more harmonic propagation into
the power system network. In addition to that, voltage fluctuation of the integrated PVDG and reverse power flow are two important
challenges to this technology. Finally, protection of power system with integrated PVDG is one of the most critical challenges to
this technology as the current protection schemes are designed for unidirectional not bidirectional power flow pattern.
1. Introduction
The growing power demand has increased electrical energy
production almost to its capacity limit. However, power
utilities must maintain reserve margins of existing power generation at a sufficient level. Currently, transmission systems
are reaching their maximum capacity because of the huge
amount of power to be transferred. Therefore, power utilities
have to invest a lot of money to expand their facilities to meet
the growing power demand and to provide uninterrupted
power supply to industrial and commercial customers [1].
The introduction of photovoltaic based distributed generation units in the distribution system may lead to several
benefits such as voltage support, improved power quality,
loss reduction, deferment of new or upgraded transmission
and distribution infrastructure, and improved utility system
reliability [2]. PVDG is a grid-connected generation located
near consumers regardless of its power capacity [3], is an
alternative way to support power demand and overcome
congested transmission lines.
The integration of PVDG into a distribution system
will have either positive or negative impact depending on
the distribution system operating features and the PVDG
characteristics. PVDG can be valuable if it meets at least
the basic requirements of the system operating perspective
and feeder design [4]. According to [5], the effect of PVDG
on power quality depends on its interface with the utility
system, the size of DG unit, the total capacity of the PVDG
relative to the system, the size of generation relative to load at
the interconnection point, and the feeder voltage regulation
practice [6].
Figure 1 shows a schematic diagram of a grid-connected
PV system which typically consists of a PV array, a DC link
capacitor, an inverter with filter, a step-up transformer, and a
power grid [5]. The DC power generated from the PV array
charges the DC link capacitor. The inverter converts the DC
power into AC power, which has a sinusoidal voltage and
frequency similar to the utility grid. The diode blocks the
reverse current flow through the PV array. The transformer
steps up the inverter voltage to the nominal value of the grid
2
International Journal of Photoenergy
PV array
DC link Inverter
Filter
Transformer
Grid
Figure 1: Schematic diagram of a grid-connected PV system.
voltage and provides electrical isolation between the PV
system and the grid. The harmonic filter eliminates the
harmonic components other than the fundamental electrical
frequency.
One of the growing power quality concerns that degrade
the performance of power systems is harmonic distortion.
The main causes of harmonic distortion are due to the
proliferation of power electronic devices like computer,
television, energy saving lamps, adjustable-speed drives, arc
furnaces, and power converters. Harmonic distortion is also
caused by nonlinearity of equipment such as transformer
and rotating machines [7]. These harmonic currents may
create greater losses in the loads which consecutively require
derating of the load, overheating of neutral conductor, overheating of transformer, and malfunction of protective devices
[8]. Another power quality problem arises at the interface
between PVDG inverters and the grid is harmonic resonance
phenomenon. Harmonic resonance phenomena will occur at
a resonant frequency where the inductive component is equal
to the capacitive component. Harmonic resonance which has
been found to be an increasingly common problem at the
interface between PVDG inverters and the grid depends on
the number of PVDG units. The effect of harmonic resonance
not only presents a severe power quality problem but also
can trip protection devices and cause damage to sensitive
equipment [9].
On the other hand, it is well known that PVDG needs
to be installed at the distribution system level of the electric
grid and located close to the load centre. Studies are usually
conducted to evaluate the impact of PVDG on harmonic
distortion, power loss, voltage profile, short circuit current,
and power system reliability before placing it in a distribution
system. To reduce power losses, improve system voltage,
and minimize voltage total harmonic distortion (THDV ),
appropriate planning of power system with the presence of
DG is required. Several considerations need to be taken into
account such as the number and the capacity of the PVDG
units, the optimal PVDG location, and the type of network
connection. The installation of PVDG units at nonoptimal
locations and with nonoptimal sizes may cause higher power
loss, voltage fluctuation problem, system instability, and
amplification of operational cost [10].
2. Power Quality Impact of PVDG
The integration of PVDG in power systems can alleviate
overloading in transmission lines, provide peak shaving, and
support the general grid requirement. However, improper
coordination, location, and installation of PVDG may affect
the power quality of power systems [11]. Most conventional power systems are designed and operated such that
generating stations are far from the load centers and use
the transmission and distribution system as pathways. The
normal operation of a typical power system does not include
generation in the distribution network or in the customer
side of the system. However, the integration of PVDG
in distribution systems changes the normal operation of
power systems and poses several problems which include
possible bi-directional power flow, voltage variation, breaker
noncoordination, alteration in the short circuit levels, and
islanding operation [2, 6]. Therefore, studies are required to
address the technical challenges caused by DG integration
in distribution systems. The interconnection device between
the DG and the grid must be planned and coordinated before
connecting any DG [12].
2.1. Harmonic Impact of PVDG. Harmonic is a sinusoidal
component of a periodic wave or a quantity which has a
frequency that is an integral multiple of the fundamental
frequency [13]. Harmonic distortion is caused by the nonlinearity of equipment such as power converters, transformer,
rotating machines, arc furnaces, and fluorescent lighting [7].
PVDG connected to a distribution system may introduce
harmonic distortion in the system depending on the power
converter technology. A power quality study was performed
on a PV system to estimate the effect of inverter-interfaced
PVDG on the quality of electric power [14]. The experimental
results indicate that the values of total harmonic distortion
THDi depend on the output power of the inverter. This
dependence decreases proportionally with reduced power
converter rating.
Another factor that influences harmonic distortion in a
power system is the number of PVDG units connected to the
power system. The interaction between grid components and
a group of PVDG units can amplify harmonic distortion [15].
In addition, PVDG placement also contributes to harmonic
distortion levels in a power system. DG placement at higher
voltage circuit produces less harmonic distortion compared
with PVDG placement at low voltage level [16]. On the
customer side, the increasing use of harmonic-producing
equipment such as adjustable speed drives may create problems, such as greater propagation of harmonics in the system,
shortened lifetime of electronic equipment, and motor and
wiring overheating. In addition, harmonics can flow back
to the supply line and affect other customers at the PCC.
Therefore, harmonic mitigation strategies for power systems
must be measured, analyzed, and identified [1].
2.2. Harmonic Resonance in a Power System with PVDG.
Resonance occurs in a power system when the capacitive
elements of the system become exactly equal to the inductive
elements at a particular frequency. Depending on the parallel
or series operation, it may form parallel or series resonance.
At a given location, when a system forms a parallel resonance,
it exhibits high network impedance, whereas for a series
resonance, it presents a low network impedance path [17].
International Journal of Photoenergy
With increasing PVDG penetration in the power grid, harmonic resonance is becoming a crucial issue in power systems
[18]. Harmonic resonance can occur at the interconnection
point of individual or multiple PVDG units to the grid
because of impedance mismatch between the grid and the
inverters. Dynamic interaction between grid and inverter
output impedance can lead to harmonic resonance in grid
current and/or voltage which occurs at certain frequencies.
The effect of harmonic resonance presents severe power
quality problems such as tripping of protection devices and
damage to sensitive equipment because of overvoltage or
overcurrent [18].
A study investigated the harmonic interaction between
multiple PVDG units and a distributed network and found
that high penetration levels of PVDG units increase harmonic emission significantly even though the PV inverters
each meet IEC 61000-3-2 specifications. Parallel and series
resonance phenomena between the network and PV inverters
were found to be responsible for unexpected high current and
voltage distortion levels in the network [19].
2.3. Effect of PVDG on Voltage Variation. The operating voltages in a distribution system are not always within required
voltage ranges because of load variations along the feeders,
the action of tap changers of the substation transformers, and
the switching of capacitor banks or reactors. This results in
voltage variations, which may be defined as the deviations of
a voltage from its nominal value [19]. Disturbances classified
as short-duration voltage variations are voltage sag, voltage
swell, and short interruption, whereas disturbances classified
as long-duration voltage variations include sustained interruption, undervoltage, and overvoltage [20].
With the growing electricity demand in distribution
systems, the voltage tends to drop below its tolerable operating limits along distribution feeders with the increase of
loads. Thus, the distribution system infrastructure should be
upgraded to solve voltage drop problems [21]. The integration
of PVDG units in a distribution system can improve the
voltage profile as voltage drop across feeder segments is
reduced because of reduced power flow through the feeder.
However, if the power generated by PVDG is greater than
the local demand at the PCC, the surplus power flows back
to the grid. The excess power from DG may produce reverse
power flow in the feeder and may create voltage rise at the
feeder [22]. Some studies investigated methods of controlling
voltage rise caused by PVDG connection into distribution
systems. Borges and Falc˜ao (2006) analyzed multiple sources
of PVDG together with the operation of voltage regulators
and concluded that the power injected by the PVDG unit
should be identified to obtain system voltages within the
allowed limits at the PVDG connection bus [23]. Chen
et al. (2012) presented two voltage control techniques in a
distribution feeder through system planning and equipment
control [24]. System planning techniques were employed in
the system design and planning stages whereas equipment
control techniques were used to regulate the bus voltages
along a feeder during real-time operation.
3
With high DG penetration at low voltage level, a violation
may occur in the upper voltage limit. Therefore, a solution
is needed to reduce the overvoltage caused by DG. Demirok
et al. (2010) addressed the overvoltage problem by applying
distributed reactive power regulation and active power curtailment strategies at the DG inverters [25]. An approach
for charging and discharging control of the storage system
(lead-acid batteries) is applied to regulate the storage capacity
effectively. An adaptive voltage control scheme which uses
an on-load tap changer and automatic voltage control relay
was proposed to increase the output capacity of DG without
violating voltage limits [24].
3. Maximum Allowable Penetration
Level of PVDG
Several studies have been conducted to investigate the
impacts of high PVDG penetration in distribution systems by
considering various constraints. Kirawanich and O’Connell
(2003) performed a simulation to investigate the harmonic
impact of a PVDG on a typical commercial distribution
system [26]. The results showed that even at the most
vulnerable lateral tap points in the system under worstcase conditions, the voltage THD did not exceed the IEEE
Standard 519 limit for up to 40% saturation of commercial
distribution system with DG units. A similar study performed
by Pandi et al. (2013) concluded that the maximum PVDG
penetration levels based on an optimal DG size and locations
on the 18-bus and 33-bus radial distribution systems are
66.67% and 33.53%, respectively [27].
Other studies focused on the maximum allowable penetration level of DG units by considering the transient stability
limit [28]. Azmy and Erlich (2005) investigated the impact
of utilizing selected DG units with different penetration
levels on various forms of power system stability [28]. The
simulation result showed that the voltage deviation decreases
significantly with 28.3% DG penetration. Moreover, it is
reported that the maximum penetration level of DG, without
violating the transient stability limit, is 40% of the total
connected load [28].
Another factor that may limit the penetration level of
DG in a typical distribution system is the steady-state voltage
rise. Celli et al. (2009) developed a method for evaluating
the critical value of DG penetration level by considering
DG siting and sizing [29]. The result showed that the limit
of the DG penetration level in a distribution system was
40% to 50%. A similar study was conducted by Chen et al.
(2012) to clarify what would happen to a distribution system
if customers were allowed to install DG units freely on
their premises and DG units became widespread [30]. The
major factor that led to overvoltage and undervoltage was
the surplus DG power in localized areas of the secondary
network, which caused the tripping of the network protectors.
Germany and Italy have a very strong PV system penetration. By 2012, the installed PV capacity reached 32 GW
and 16 GW, respectively. More than 20% of the capacity
installed is connected to the distribution voltage network.
In Germany—specifically—63% of the PVDG is connected
4
to the household voltage level. The integration of this big
amount of PVDG makes challenges to the power system. For
example, in case of have a large share of PVDG units that
switch off simultaneously may increase the grid frequency
up to 50.2 Hz. However, this problem has been addressed
in Germany by requiring the back-fitting of installations
with a nominal power above 10 kW. In addition to that,
voltage regulation within tolerable limits is another challenge
faced. Storage has been identified as a possible solution
for providing flexibility to the power system and could
possibly generate value streams from flexibility. However the
feasibility of such a solution is remains challenging. A number
of authors assess how storage could overcome the technical
challenges of PV integration. For example, the battery can be
optimally sized in order to avoid overvoltage caused inverter
disconnection [31–36].
4. Optimal Placement and Sizing of PVDG
Voltage variation and harmonic distortion are two major
disturbances in distribution systems. The voltage drop occurs
because of increasing electricity demand, thereby indicating
the need to upgrade the distribution system infrastructure.
Studies have indicated that approximately 13% of the generated power is consumed as losses at the distribution level
[37]. To mitigate voltage variation and harmonic distortion
in distribution systems, several strategies were applied, such
as the use of passive and active power filters to mitigate
harmonic distortion and the application of custom power
controllers to mitigate voltage variation problems. However,
these mitigation strategies require investment. Therefore, to
improve voltage profile and eliminate harmonic distortion
in a distribution system with PVDG, a noninvasive method
is proposed, which involves appropriate planning of PVDG
units and determining optimal placement and sizing of
PVDG units.
Before installing PVDG units in a distribution system,
a feasibility analysis has to be performed. PVDG owners
are requested to present the type, size, and location of
their PVDG [27]. The power system is usually affected by
the installation of PVDG. Therefore, the allowable PVDG
penetration level must comply with the harmonic limits.
Thus, optimal placement and sizing of DG is important
because installation of DG units at optimal places and with
optimal sizes can provide economic, environmental, and
technical advantages such as power losses reduction, power
quality enhancement, system stability, and lower operational
cost [11].
Several methods have been applied to determine the
optimal location and size of PVDG in a distribution system.
The analytical method used for optimal PVDG placement
and sizing is only accurate for the model developed, and it
can be very complicated for solving complex systems. The
power flow algorithm [10] has been used to find the optimum
PVDG size at each load bus by assuming that each load bus
can have a PVDG unit. However, this method is ineffective
because it requires a large number of load flow computations.
Analytical methods can also be used to place the PVDG
International Journal of Photoenergy
in radial or meshed systems [38]. In this method, separate
expressions for radial and meshed systems are required,
and complex procedures based on the phasor current are
applied to solve the PVDG placement problem. However, this
method only determines the optimum PVDG placement and
not the optimum PVDG size as it considers a fixed PVDG
size.
The metaheuristic method is also used in optimal placement and sizing of DG in distribution systems. This method
applies an iterative generation process which can act as a
lead for its subordinate heuristics to find the optimal or
near-optimal solutions of the optimization problem [39].
It combines different concepts derived from artificial intelligence to improve performance. Some of the techniques
that adopt metaheuristics concepts include genetic algorithm
(GA), Tabu search, particle swarm optimization (PSO), ant
colony optimization (ACO), and gravitational search algorithm (GSA).
The implementation of the general optimization technique for solving the optimal placement and sizing of PVDG
problem is depicted in Figure 2. A multiobjective function
is formulated to minimize the total losses, average total
voltage harmonic distortion, (THDV ) and voltage deviation
in a distribution system. The procedures for implementing
the general optimization algorithm for determining optimal
placement and sizing of PVDG are described as follows.
(i) Obtain the input network information such as bus,
line, and generator data.
(ii) Randomly generate initial positions within feasible
solution combination, such as the PVDG location,
PVDG size in the range of 40% to 50% of the total
connected loads, and PVDG controllable bus voltage
in the range of 0.98 p.u to 1.02 p.u.
(iii) Improvise the optimization algorithm using the optimal parameters such as population size, number of
dimension, and maximum iteration.
(iv) Run loadflow and harmonic loadflow to obtain the
total power loss, average THDV and voltage deviation.
(v) Calculate the fitness function.
(vi) Check the bus voltage magnitude and THDV constraints. If both exceed their limits, repeat step (iv).
(vii) Update the optimization parameters.
(viii) Repeat the process until the stopping criteria are
achieved and the best solution is obtained.
5. Conclusion
This paper describes an overview of the relevant aspects
related to PVDG and the impacts it might have on the
distribution system. This paper evolves the background of
PVDG and its impacts on power quality and the maximum allowable penetration level of PVDG connected to
a distribution system. The implementation of the general
optimization technique for solving the optimal placement
and sizing of PVDG problem with multiobjective functions
International Journal of Photoenergy
5
Start
Getting
network
information
Initial population
generation
IGSA algorithm
Iteration = 1
Iteration + 1
Improvise IGSA
parameter
Update velocity and
position
Calculate the MATPOWER loadflow
and the harmonic loadflow
Calculate F, Kbest
and 𝛼
No
THDvi ≤ THDvmax ?
Vmin ≤ Vi ≤ Vmax ?
𝜆=1
Yes
Yes
Calculate the fitness
value
Population = max?
𝜆=0
New agent
M(k) > M(i)
and Rik ≤ 𝜏
No
Calculate G, M, best
and worst
No
Calculate choatic
sequence
No
Yes
Iteration = max?
Yes
Best solution
End
Figure 2: Flowchart of the general optimization technique for determining optimal placement and sizing of PVDG in a distribution system.
such as minimization of losses, THDv, and voltage deviation
is explained. The multiobjective functions are considered as
the technical benefits factors for optimal planning PVDG in
the distribution system. It is concluded that there are great
opportunities and benefits such as technical, economical, and
environmental benefits are offered when installing PVDG
in a distribution system. However, there are some technical
issues and challenges come up when incorporating PVDG in
a distribution system. These issues and challenges concluded
by several researchers need to be assessed for proper PVDG
planning and operation in the distribution network. These
issues are as follows.
(i) Reliability: the amount of power produced by renewable energy source based PVDG is not consistent like
wind and solar source. Thus, the high penetration of
PVDG into grid can decrease the reliability of the
power system network.
6
International Journal of Photoenergy
(ii) Power quality: the high penetration of PVDGs can
lead to more harmonic propagation into the power
system network, increase the losses, and possibly
decrease the equipment life time.
(iii) Voltage fluctuation: it is a significant issue for high
penetration level of PVDG. This issue desires to be
critically considered in integrate inconsistent sources.
For example, the fluctuation of solar source in supplying power to the load will caused overvoltage
or undervoltage. The voltage fluctuation is very bad
impact on the sensitive equipment.
(iv) Reverse powerflow: incorporating PVDG in the system causes malfunctions of protection systems as they
are configured by the unidirectional form.
(v) System frequency: the unbalances between supply
and demand will result to the deviations from the
system nominal frequency. The high penetration of
PVDG affects system frequency and makes the process of control more complicated.
(vi) Protection schemes: the common distribution networks are configured in the radial form. Thus, the
protections system is designed accordingly to the
unidirectional flow patterns. However, the integrating
of PVDG changes the flow into bidirectional and
needs additional safety equipment and resizing of the
network such as grounding, short-circuit, breaking
capacity, and supervisory control and data acquisition
(SCADA) systems.
(vii) Islanding protection: anti-islanding protection
schemes presently implement the PVDGs to remove
immediately for grid faults through loss of grid
(LOG) protection system. This significantly decreases
the advantages of PVDG deployment. For avoiding
disconnection of PVDGs during LOG, several islanding protection schemes are being developed. The
biggest challenge for the islanding protection schemes
is the protection coordination of distribution systems
with bidirectional fault current flows.
Conflict of Interests
The authors hereby confirm that there is no conflict of
interests in the paper with any third party.
Acknowledgments
This work is supported by Universiti Teknikal Malaysia
Melaka under research Grant number PJP/2013/FKE (24C)/
S01255. Part of the authors is supported by the research cluster
Lakeside Labs funded by the European Regional Development Fund, the Carinthian Economic Promotion Fund
(KWF), and the state of Austria under Grant 20214|22935|
34445 (Project Smart Microgrid).
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